Field
The present invention relates to MEMS sensors and especially to MEMS sensors for measuring linear acceleration as defined in independent claim 1.
Description of the Related Art
Micro-Electro-Mechanical Systems, or MEMS can be defined as miniaturized mechanical and electro-mechanical systems where at least some sensor elements have a mechanical functionality. MEMS structures can be applied to quickly and accurately detect very small changes in physical properties.
MEMS devices convert a measured mechanical signal into an electrical signal. MEMS sensors measure the mechanical phenomenon and the electronics then process the information derived from the sensors and through some decision making capability direct the actuators to respond by e.g. moving, positioning, or regulating in order to thereby control the environment for some desired outcome or purpose. MEMS devices might be capacitive or make use of piezoelectric transduction.
In an inertial sensor employing a MEMS-type accelerometer, structures like rotor masses, cantilevered beams and/or interdigitated comb fingers can be used to sense displacement of these structures.
The two major components of a MEMS accelerometer are the rotor mass and the sensing electrode pairs. The rotor mass and the sensing electrode pairs are anchored to a substrate. Since the rotor mass is suspended over a recess on the substrate with springs, it is free to move in response to an external acceleration. When external forces are applied to the accelerometer, the rotor mass moves against the forced direction due to an inertial force. The movement causes capacitance variations between interdigitated comb fingers which form pairs of parallel plate capacitors.
In a capacitive MEMS accelerometer, a variable capacitor is formed between a stationary electrode and a movable electrode attached to a suspended inertial rotor mass.
Acceleration sensors sense the acceleration force and detect the movement of the inertial rotor mass displacing itself elastically under the effect of acceleration and thereby also the movable electrode deflects in response to the acceleration in an accelerometer. The change in capacitance, which directly shows as a change in distance (or gap) between the comb finger electrodes is related to the displacement of the rotor mass.
The deflection is sensed by associated electronics and converted to an electrical signal, which is then delivered by the electronics to an external computer. The computer processes the sensed data to calculate the property being measured.
An accelerometer is a device that measures proper acceleration, a.k.a. the g-force. Proper acceleration is the physical acceleration experienced by an object, and it's measured relative to an inertial observer (an inertial frame), who is momentarily at rest relative to the object being measured. For example, an accelerometer at rest relative to Earth's surface will indicate about 1 g acceleration upwards. In order to obtain the acceleration due to motion with respect to the Earth, gravity offset shall be subtracted from the readings, and corrections are needed for effects caused by the Earth's rotation relative to the inertial frame. A multi-axis accelerometer detects magnitude and direction of the proper acceleration as vector quantity, and it may be used to sense for example orientation, coordinate acceleration, vibration, shock and falling in a resistive medium.
2-axis accelerometers measure acceleration in two directions and 3-5 axis accelerometers measure acceleration in three directions. There are also 1-axis accelerometers that measure acceleration in one direction.
The measurement range of an accelerometer is the level of acceleration supported by the sensor's output signal specifications, typically specified in ±g. This is the greatest amount of acceleration the part can measure and accurately represent as an output. An inclinometer is an instrument for measuring angles of slope, elevation or depression of an object with respect to gravity. An accelerometer, such as a MEMS accelerometer, may be used as a sensor for an inclinometer, when suitable calculation is provided for converting the detected acceleration values into angle values. The axes of measurement of an inclinometer are typically, but not necessarily, orthogonal.
A tilt sensor or a tilt meter is an instrument measuring tilt, often with respect to a plane defined by two axes. A typical tiltmeter is designed for measuring changes from the vertical level. Full motion tilt sensor may use at least three axes. An accelerometer capable of measuring acceleration may be used as a sensor for a tiltmeter, when suitable calculation is provided for converting the detected acceleration values into resultant vector angle values. An accelerometer may measure tilt with respect to one, two or three axes. The axes of measurement of a tiltmeter are typically, but not necessarily, orthogonal.
We will use a common term inclinometer for any device capable of measuring inclination or tilt.
Depending on purpose of use, inclinometer offset stability requirements may be stringent, requiring mechanically very stable MEMS accelerometer sensor elements. Anchoring, in other words suspension of the accelerometer sensor elements on an anchor structure in the substrate is a key factor for stability of MEMS sensor elements. In order to ensure that any mechanical stress through the package of a MEMS sensor element causes minimal errors, a well-known method is to place stator and rotor anchor structures near to each other. If mechanical stress occurs, it will deform stator and rotor similarly, which compensates offset caused by such deforming. Offset stability can also be improved by using multiple detection cells in each dimension in order to enable double differential self-compensation. However, if anchors of individual sensor elements are far from each other, self-compensation is not perfect.
U.S. Pat. No. 7,322,242 presents a micromechanical structure with a centroidal rotor-anchoring region coupling a single, frame-like rotor and four stator structures arranged with stator-anchoring regions near the rotor anchoring region. The problem relating to this prior art is that although use of a single rotor enables differential detection, the device may also be subject to common mode errors that are not detectable with such single rotor mass solution. An improved MEMS sensor design is needed that enables double differential detection, which also enables canceling of common mode errors.